The prior art has sought for many years to incorporate electrically conductive fibers or threads into fabric, for various desired applications, both military and commercial. What is essentially desired is an insulated, electrically conductive fiber or “microwire” of between 0.0004-0.004 inches, that is, 10-100 microns, in diameter. Ideally the diameter of the microwires would be less than 25 microns, that is, no greater than 0.001 inches. Further desired characteristics are that the resistance of the conductive component of the fiber per unit length be no more than about five times that of copper, to ensure adequate electrical performance, that the diameter of the central conductor be about 60% of the overall fiber diameter, and that the microwire is suitably flexible to be processed into a wearable textile product and sufficiently durable to withstand ordinary use in a garment. Such microwires are contemplated for carrying heating current, carrying data, for providing electromagnetic shielding, for antenna and sensor fabrication, for connection of electronic components secured to the fabric of a garment, and for other uses.
Mitamura et al U.S. Pat. No. 5,173,366 shows apparatus and methods for manufacture of fine conductive fibers, that is, less that 50 microns in overall diameter. These are made by coprocessing a low-melting point conductive metal core in a sheath of a higher-melting point polymer, which is also a feature of the present invention. However, Mitamura does not suggest that the fibers formed according to the teachings of the patent would be suitable as conductors, but acknowledges that the conductive core fibers have significant discontinuity of 5 cm or less per meter. These discontinuities greatly limit the utility of the Mitamura fibers. Moreover, the specific resistance of the Mitamura fiber is “about 104 Ω-cm” (col. 3, line 60); this is far too high for many purposes.
It is accordingly an object of the invention to provide extremely fine microwires, that is, wires comprising a conductive core and an insulative sheath, in which the conductors are continuous and their conductivity is sufficiently high that the microwires are suitable for service as conductors in electronic circuits.